Method and apparatus for cleaning a mobile immersed structure

ABSTRACT

The invention concerns a cleaning process for an immersed and towed seismic streamer ( 1 ), characterised in that a cleaning appliance ( 11 ), including a tool for cleaning the seismic streamer, is engaged around the seismic streamer, together with a hydrodynamic drag structure which results in the free movement of the cleaning appliance along the length of the seismic streamer ( 1 ).

This present invention relates to a process and a cleaning appliance fora mobile immersed structure (or floating if appropriate).

Seismic arrays or “streamers” are used in particular to exploreunderwater ground structures, in particular to look for the presence ofoil or of gas in the sedimentary layers of the ocean floor.

Such a streamer generally includes a hollow tubular structure, along thelength of which are provided hydrophones (sensors which are sensitive tounderwater acoustic waves), as well as vanes (“or planes”) which areused to adjust the depth of immersion of the section of streamer towhich they are attached. The hydrophones are used to measure thepropagation times of echoes of sound pulses which are returned by thesedimentary layers.

Streamers can have a length of several kilometres or even tens ofkilometres. They can remain immersed in the seawater for severalconsecutive months. They are generally immersed to a shallow depth (of afew metres), and towed along at low speed (not more than 5 knots). Thetubular structure of the streamer is therefore subject to fouling.Living organisms that develop on the outer surface of this structure canlead to the formation on the latter of amalgams or incrustations whichinterfere with the measurements to be performed.

It is therefore necessary to clean the outer surface of the tubularstructure of the streamer. This cleaning necessitates cessation of themeasurements and “rewinding” of the streamer on board the ship that istowing it. This results in a considerable wastage of time.

One objective of the invention is to offer a process and a cleaningappliance for such a structure, which provides at least a partial remedyfor these drawbacks.

According to a first aspect, the invention consists of cleaning aseismic streamer while it is being towed by a ship, by means of acleaning appliance consisting of a tool for cleaning the outer surfaceof the streamer, and a structure which creates a hydrodynamic drag byfree displacement of the cleaning appliance along the length of thestreamer.

It is preferable that one should generate movement of the cleaningappliance along the length of the streamer at a speed of at least 0.5metres per second, and in particular at a speed located within a rangeof 1 to 2.5 m/s. To this end, the streamer is towed at a higher speed inorder that the cleaning appliance moves at an adequate speed in relationto the water. The ratio of the towing speed of the streamer to thesliding speed of the cleaning appliance along the length of the streameris of the order of 1.5 m/s for example, with the speed of the cleaningappliance in relation to the water being of the order of 1 m/s forexample.

According to another aspect, the invention proposes a cleaning appliancefor the cleaning of a seismic streamer—or a similar rope-like immersedstructure—which includes:

-   -   a tool for cleaning the outer surface of the streamer,    -   the means of positioning and guidance, adapted to the shape of        the streamer, in order to allow the cleaning appliance to remain        attached to the streamer and to travel along the length of the        latter, and    -   a drag structure to create hydrodynamic resistance during        movement of the cleaning appliance along the longitudinal axis        of the streamer (and/or along the axis of the positioning and        guidance devices), so as to give rise to a movement of the        cleaning appliance along the streamer when the latter is moved        (towed under the water) along its longitudinal axis.

Because of the presence of the drag structure, it is possible to causethe cleaning appliance to slide along the length of the immersedstreamer when it is towed, without requiring the provision of, or areserve of, external drive energy. The cleaning appliance movesindependently by sliding along the length of the tubular structure ofthe streamer. No “umbilical” connecting the appliance to the ship istherefore necessary.

It is preferable that the resources for positioning and guidance of theappliance in relation to the streamer should include a pair—andpreferably several pairs—of wheels, rollers, runners, or pulleys, toroll or slide at low friction, respectively on two approximatelydiametrically-opposite portions of the outer surface of the tubularstructure of the streamer.

It is preferable that the appliance should also include—and this is yetanother aspect of the invention—some buoyancy resources in order toallow the cleaning appliance to maintain an approximately constant tilt,and therefore an approximately constant orientation (to within plus orminus 5 or 10 degrees, for example) in relation to the longitudinal axisof the streamer, during its movement along the length of the towedstreamer. This allows the appliance to be astride the vanes—“wings” or“planes” or other protruding accessories—which are attached to thetubular structure of the streamer, during the movement of the cleaningappliance along the length of the streamer.

According to another aspect of the invention, a cleaning appliance for aseismic streamer according to the invention includes a mobile cleaningtool, in particular a rotary cleaning brush, as well as the driveresources to move the tool (in particular to cause the brush to rotate),under the effect of movement of the cleaning appliance in relation tothe water which surrounds it and/or in relation to the streamer. In thislatter case, these drive resources are preferably moved by friction onthe outer surface of the rope-like (tubular) structure of the streamer.

Implementation of the drag structure can take various forms. Thedifferent elements making up the appliance, which project in relation tothe outer surface of the streamer, can in certain cases contribute tothe creation of a structure presenting a level of drag which issufficient, firstly, to oppose the mechanical friction forces due to thecleaning appliance sliding along the length of the streamer and,secondly, to provide it with an adequate speed of movement along thelength of the latter. However, in order to limit the size of theappliance and also the acoustic noise resulting from of the movement ofthe appliance, it is preferably equipped with a specially-designedstructure in order to increase the hydrodynamic drag of the cleaningappliance.

According to a preferred method of implementation, this structureincludes a section of conduit which broadens out at the front and isopen at its two ends. This results in a structure in the form of athroat or funnel or water sock, through which the surrounding seawaterflows because of the movement of the appliance in relation to thelatter.

When such a drag structure is used, the drag results from the high dragcoefficient of a cone placed in the direction opposite to the flow.

Alternatively or additionally, it is possible to equip the cleaningappliance with a flap or pivoting deflector capable of extendingapproximately crosswise in relation to the direction of movement of thecleaning appliance, in order to give rise to a additional drag in anautomatic manner.

It is preferable to equip the cleaning appliance with a drag structureconsisting of two drag structure elements, which are approximatelyidentical (or symmetrical) in shape and dimensions and positionedapproximately diametrically opposite to each other on the axis ofmovement of the appliance along the streamer, that is in relation to thelongitudinal axis of the streamer. This enables one to orientate aresultant of the two drags contributed respectively by each dragstructure element, approximately parallel to the axis of movement of thecleaning appliance along the streamer. This also allows the hydrodynamicforces exerted on the cleaning appliance and on the streamer to bebalanced. The result is a reduction in the mechanical stresses exertedby the cleaning appliance on the streamer as well as greatereffectiveness of the system for “passive propulsion” of the appliancealong the streamer.

In order to adapt the cleaning appliance to various towing conditions ofthe streamer, and in particular in order to increase or reduce the speedof movement of the appliance along the streamer, the latter preferablyincludes the means to adjust the drag coefficient of the drag structure.To this end, part at least of the drag structure can be arranges topivot or to move between a position of maximum drag and a position ofminimum drag.

In order to reduce the forces exerted on the streamer and to facilitatethe clearance, by the cleaning appliance, of projections presented alongthe length of the tubular body of the streamer, the positioning and/orguidance devices of the appliance on the streamer are preferablydeployable and/or retractable. To this end, the appliance is preferablyequipped with the means to force the positioning and/or guidance devicesto a position pressing onto the outer surface of the streamer, whilealso allowing the positioning and/or guidance devices to move away fromthe outer surface to a position where obstacles are cleared. Accordingto a preferred method of implementation, the appliance includes asuspension arm supporting the positioning and/or guidance devices (suchas wheels or rollers), as well as a spring for the return of thesuspension arm to a position where the positioning and/or guidancedevices are pressing onto the outer surface of the streamer.

Other advantages and characteristics of the invention will be understoodby reading the following description, which refers also to the appendeddrawings, illustrating the preferred methods of implementation of theinvention, without any limiting character.

FIG. 1 is a schematic view illustrating the implementation of a cleaningappliance according to the invention for the cleaning of a seismicimmersed streamer in the process of being towed.

FIG. 2 is a rear view in perspective of a preferred method ofimplementation of a cleaning appliance according to the invention,engaged around a portion of seismic streamer.

FIG. 3, which is a view along line III of FIG. 2, illustrates the samecleaning appliance as seen from the rear.

FIG. 4 illustrates the same cleaning appliance as seen from the side,and is a view along line IV of FIGS. 2 and 3.

FIG. 5 is a plan view of the appliance, and a view along line V of FIG.3.

FIG. 6 is a front view, in perspective, of the appliance of FIGS. 1 to5.

FIG. 7 is a rear view of the appliance.

FIG. 8 is a bottom view of the appliance of FIGS. 1 to 7, and is a viewalong line VIII of FIG. 7. Unlike FIGS. 1 to 5, the streamer is notshown on FIGS. 6 to 8.

FIG. 9 illustrates, in perspective, a brushing module that can be drivenby friction against the streamer, which equips the appliance of FIGS. 1to 8.

FIG. 10 illustrates, in perspective, one of two drag structures of theappliance of FIGS. 1 to 8.

In this present application, the terms “front” and “rear” are employedwith reference to an imaginary observer located behind the ship, inparticular behind the streamer, and observing the stern of the ship thatis towing the streamer.

With reference to FIG. 1, a streamer 1 is towed by a ship 2 to which itis attached by means of the drum 3 of a winch fitted to the ship. Thestreamer is designed to be used to analyse the structure of the seabottom 4 by means of the resources (not shown) for processing thesignals delivered by the sensors on the streamer. To this end, itincludes a multiplicity of hydrophones 5 attached to a flexible tube 6,a front end 7 of which is attached to the winch, and a rear end 8 whichis free. The streamer is towed in the direction of the arrow 9 at aspeed of approximately 5 knots. The streamer also includes buoyancydevices 10 attached to the tube 6 in order to hold the streamer at auniform depth (over its length) and/or constant depth (over time).

As illustrated in particular in FIGS. 2 to 5, the appendages 10 on thestreamer can include an upper part 10 a acting as a buoyancy device, alower part 10 b approximately diametrically opposite to the upper part10 a, and two lateral parts (vanes or planes) 10 c and 10 d extendingfrom the lower part 10 b.

On these figures, the streamer cleaning device 11 is illustrated in aposition in which it is astride the appendage 10 attached to the tubularbody 6 of the streamer. The tubular body 6 extends along itslongitudinal axis 12, the direction 9 of traction of the streamer beingapproximately parallel to axis 12. In the position where the cleaningappliance 11 is astride the streamer, the axis of symmetry 14 of themeans for positioning and guidance of the appliance is approximatelycoincident with the longitudinal axis 12 of the tubular part 6 of thestreamer. As illustrated in particular in FIGS. 3 and 7, the appliance11 presents a general symmetry along a front-to-back plane 13 containingthe axis of symmetry 14 of the positioning and guidance resources.

With reference to FIGS. 2 to 8, the cleaning appliance 11, forming atrolley running along the length of the streamer, includes:

-   -   a chassis 15 consisting of two transverse shackles 16 and 17        connecting two rails 18 and 19 extending parallel to axis 14, as        well as an upper middle structure 20 designed to facilitate the        lifting of the appliance 11;    -   two hollow longitudinal elements 21 and 22 attached to the upper        part of the chassis 15 by bolts 23 positioned on the outside of        the shackles 16 and 17 and used to provide the immersed cleaning        appliance 11 with an apparent mass (or buoyancy potential) of        approximately zero and an approximately constant tilt;    -   two blades 24 positioned at the rear end of the appliance 11 and        intended to cut or knock off large incrustations formed on the        outer surface of the tubular structure 6 of the streamer;    -   a pair of front guidance rollers 25, in the form of a diabolo,        the profile of which is complementary to the cylindrical surface        6 of the streamer, and which are designed to rotate along two        axes 27 parallel to the plane of symmetry 13 and perpendicular        to axes 12 and 14, on arms 29 designed to pivot on rails 18 and        19;    -   two rear positioning and guidance rollers 26, identical or        similar to the front guidance rollers 25, which are designed to        pivot along two axes 28 parallel to plane 13 and to axes 27, on        arms which pivot on rails 18 and 19;    -   two intermediate rollers 30, also positioned on either side of        plane 13 and symmetrical in relation to the latter, which are        held in contact with the structure 6 by arms fitted with the        means of retraction, and attached to rails 18 and 19. These        intermediate rollers 30 are employed to drive in rotation, by        means of belts or transmission chains 31, two rotating brushes        32 also positioned symmetrically on either side of plane 13 and        of the longitudinal axis 14 of the appliance;    -   two structures 33 used to increase the hydrodynamic resistance        of the appliance, which are attached on either side of the        latter, symmetrically in relation to plane 13 and to rails 18        and 19, by fixing bolts 34.

With reference to FIG. 9, the device for rotation of the rotatingbrushes 32 along their axis of rotation 41 parallel to front-to-backplane 13 of the appliance, includes rollers 30 which are driven byfriction against the outer surface of the tubular structure 6 of thestreamer, in rotation along their axis 40 parallel to axis 41. Theroller 30 pivots on its axis in relation to a yoke 42 which slides alongan axis 43 in relation to a shaft 44, which is fixes to one of rails 18and 19 of the appliance by means of an attachment element 45 and bymeans of bolts 49. The yoke 42, acting as a bearing to the frictionroller 30, is held against the outer surface of the streamer 6 by ahelical return spring 46 on axis 43. The brush 32 rotates on its axis 41between two blades 48 linked by a spacer 47. The elements 47 and 48 usedto support the brush 32, the friction roller 30 and the drive belt ofthe brush 32 from the roller 30, are designed to move in relation to oneof the rails, and form a suspension arm for the roller 30 and the brush,allowing these elements 30 and 32 to be held against the outer surfaceof the streamer. The spring suspension can be enhanced by the additionof a damper 70 connecting the yoke 42 to the element 45 on the chassis.The brush 32 is moved by the runner 30 by means of a belt (not shown),with a speed of rotation which is greater than that of the runner.

With reference to FIG. 10, the structure (flared at the front) for thegeneration of drag 33 is essentially composed of a metal structure withtwo parallel blades 50 and 51, which are linked by an incurving element52, with elements 50 to 52 capable of being created by the curvature ofa metal plate. Each of the blades 50 and 51 is drilled out with orifices53 allowing the attachment by bolts 34 of the structure 33 to the one ofthe rails 18 or 19 of the appliance.

The structure 33 also includes an approximately plane shutter 54extending transversally in relation to the blades 50 and 51 and fixed tothe latter by a bolt 55. The orientation of the shutter 54 in relationto the structure 50 to 52 can be modified by pivoting the shutter 54 inthe direction of the arrow 56 along the axis 57 of the bolt 55. Thestructure 33 also includes an approximately conical part 58 forming anextension to parts 50 to 52, with the conical part 58 itself beingextended, as illustrated in particular by FIG. 2, by a cylindrical part59 which is open at its rear end 60.

The transverse section for the passage of water at the front 61 of thecleaning appliance is significantly greater than the intermediatesection in the middle part 62 of the appliance 33, in particular becauseof the inclination of the shutter 54 in relation to axis 14 of theappliance. The section allowing the passage of water is reduced stillfurther in the exit conduit 59 of the structure 33.

Where appropriate, the shutter 54 can be designed to rotate freely onaxis 57, its inclination being limited by a return spring (not shown),with the movements of the shutter possibly being damped with a dampingdevice (not shown).

In the mounted position of the structure 33 onto the appliance 11, thelatter being forced through in the water in the direction of the arrow 9(FIGS. 4 to 6) by the streamer at a speed which is less than that of thestreamer because of its motion along the length of the latter, the wateris taken in at the front section 61 of the structure 33, traverses themiddle section 62, the conical portion 58 and the cylindrical rearportion 59, before escaping via the orifice 60. The force exerted by thewater on the appliance 11 and on the structure 33 in particular, causesthe cleaning appliance to move along the streamer, from its front(reference 7, FIG. 1) to its rear end (reference 8, FIG. 1), where thecleaning appliance 11 can remain pressed against an end-stopincorporated into the streamer until completion of a measurement run.

In order to facilitate the placement of the appliance around the frontend of the streamer, before cleaning the latter, the chassis 15 isdivided into two parts (left and right) which are assembled by the meansof an attachment and locking device (reference 80, FIG. 5) afterengagement of the guidance wheels and the cleaning tools against theopposite surfaces of the tubular body 6 of the streamer.

1. A process for the cleaning of a towed (9) seismic streamer (1, 6)wherein a cleaning appliance (11) including a tool (24, 32) for cleaningthe seismic streamer (1), in addition to the hydrodynamic drag structure(22), is engaged around the seismic streamer (1), and in that theseismic streamer is then towed so that the drag created by the structure(33) has the effect of moving the cleaning appliance along the length ofthe seismic streamer.
 2. A cleaning process according to claim (1), inwhich the speed of movement of the applicance along the seismic streameris located in a range of between 0.5 and 2.5 meters per second.
 3. Anappliance (11) for the cleaning of an immersed or floating seismicstreamer (1, 6) wherein it includes: a tool (24, 32) for cleaning theseismic streamer, resources (25, 26) for positioning and guidance of theappliance along the length of the seismic streamer, resistance resources(33) which, where the streamer is towed, create a hydrodynamic drag thatis sufficient to overcome the friction forces and allow the cleaningappliance to move along the length of the seismic streamer (1, 6).
 4. Acleaning appliance according to claim 3, which includes the driveresources to operate the cleaning tool by movement of the appliancealong the seismic streamer.
 5. A cleaning appliance according to claim4, in which the cleaning tool includes at least 2 brushes, each brushbeing driven by a rotating roller (30) driven by friction against theseismic streamer (1, 6).
 6. A cleaning appliance according to claim 3,which includes a blade (24) for removal of the incrustations attached tothe seismic streamer (1,6).
 7. A cleaning appliance according to claim 3in which the positioning guidance devices of the appliance in relationto the streamer consist of a pair- and preferably of several pairs- ofwheels, rollers, runners, or pulleys (25, 26), to roll or slide at lowfriction, respectively on two approximately diametrically oppositeportions of the outer tubular surface of the seismic streamer.
 8. Acleaning appliance according to claim 3 which also includes buoyancyresources (21, 22) in order to allow the cleaning appliance to maintainan approximately constant position during its passage along the lengthof the towed streamer, and therefore an approximately constant tilt inrelation to the longitudinal axis (12) of the streamer, and to enablethe cleaning appliance to present an approximately zero buoyancy inorder not to interfere with the balance of the streamer.
 9. A cleaningappliance according to claim 3 in which the resistant resources consistof two drag structures (33) in the form of a throat, funnel, deflectoror water sock, or approximately identical shape and dimensions,positioned symmetrically in relation to a central front-to-back plane(13) and/or in relation to the guidance axis.
 10. A cleaning applianceaccording to claim 3 which includes a drag structure (33) and resources(54, 55) for adjustment of the drag coefficient of the structure (33).11. A cleaning appliance according to claim 3 which includes the means(46, 48, 70) for suspension of resources (25, 26) in relation to thechassis (15) of the cleaning appliance.